A floppy disk and a CD-ROM (Compact Disk-Read Only Memory) are conventionally known as an information recording medium for program data, audio information and video information. In particular, the CD-ROM is widely used for the distribution of various softwares because of its large capacity of more than 600 MB and low production cost.
On the other hand, the promotion of high speed of a personal computer in recent years has increased the demand for outputting video and audio data (hereinafter, referred to as AV data) on the personal computer. For example, a digital data file where data is compressed by video image compression system called MPEG 1 (Moving Picture Experts Group) is recorded on CD-ROM, and an application for distributing such a CD-ROM has been increasingly available. However, since the MPEG 1 system generally compresses video data which requires a large capacity at a high compression ratio, quality of video images is significantly degraded. Therefore, the MPEG 1 is unsuitable for use where high quality in video images is required, such as movies or the like.
In recent years, development for recording high quality video data on an optical disk having a large capacity close to 5 GB, using a higher performance video image compression system called MPEG 2 system has been pursued. Capitalizing on the large capacity, the optical disk called DVD (Digital Video Disk) can record high quality digital AV data for two hours or more, and thus greatly expected as an AV data recording medium of the next generation. On the other hand, the DVD is expected to allow for reproduction of high quality AV data on a personal computer by a DVD drive for reproducing DVD in connection to the personal computer. In addition, the DVD is expected to substitute for the CD-ROM as a medium for distributing calculator software.
However, when the DVD drive as a peripheral device for the personal computer, commercially available, digital data recorded in the DVD may be output to the personal computer, and may easily be copied on a rewriting type medium, such as a hard disk nor an MO (Magneto-Optical Disk). Easy copying of the digital AV data causes problems such as illegal copying of the AV data recorded in the DVD without the permission of the copyright holder and distribution of altered AV data. This makes it extremely difficult to protect the copyright. This brings about a disadvantage not only to the copyright holder of the data, but also possibly to users in the case where the copyright holder sets the price in view of unauthorized copies being made, or does not produce disks for fear of alteration of the data. Hereinafter, the problem described above is referred to as a first problem.
On the other hand, as uses for the information recording medium where the AV data is recorded, a variety of uses are considered. In some uses among these uses, the fact that the information recording medium can be reproduced in any reproducing device causes a problem. In such a use, it is preferable to differentiate a reproducing device capable of reproducing and a reproducing device incapable of reproducing. For example, for disks generally called "karaoke" disks where video data including words corresponding to reproduced music is recorded, there exist two types of disks: disks privately used in households (hereinafter, referred to as consumer disks) and disks used in a facility such as where customers enjoy karaoke for a predetermined charge (hereinafter, referred to as industrial disks). Since the industrial disks are produced under the premise that they are supplied in a large quantity to limited users, they are supplied at a low price. On the other hand, consumer disks are sold at a high price due to individual sale.
However, in the case where the industrial disks and the consumer disks have entirely the same format, there is a possibility of selling the industrial disks as the consumer disks at low price in the consumer market. This prevents the consumer disks from being distributed in a proper price in the market, thus resulting in a disadvantage for disk manufacturers and users who purchase the consumer disks through the normal route. Therefore, it is desirable to differentiate the reproducing device capable of reproducing the consumer disks from that capable of reproducing industrial disks in such a use. For another example, a disk having the content which causes an ethical problem may be reproduced. The criterion which determines whether or not the disk is ethical is varied from country to country. Therefore, a disk to be reproduced in a certain country may not be desired to be reproduced in another country. Therefore, a mechanism is required which allows a disk which causes an ethical problem to be reproduced only in a specific country where the sale thereof is permitted. As described above, there has been a problem that the reproducing device capable of reproduction and the reproducing device incapable of reproduction cannot be differentiated depending on the use. Hereinafter, this problem is referred to as a second problem.
As one means for solving the two aforementioned problems, there is a method of scrambling (encrypting) data to be recorded in the information recording disk for recording. More specifically, regarding the first problem, at the time of a copying operation in the personal computer, data scrambled based on a key is transmitted back, and a key for descrambling is not transmitted back, thus preventing the copying operation (the copying operation is performed, but since the descrambling is not effected, the copying operation makes no sense).
Furthermore, regarding the second problem, it is possible to classify a device capable of descrambling and a device incapable of descrambling by producing disks which are differently scrambled depending on the contents of the disks. Thus, the scrambling (encrypting) of the recorded data is effective on the two aforementioned problems. However, another problem is a method for descrambling data or how to specify a key for descrambling data.
As a first conventional example for encrypting in a data field, a system of recording an encrypting key in a main data field of a sector different from an encrypted data sector was proposed in a CD-ROM in FIG. 3 of Japanese Laid-Open Patent Publication No. 7-249264. In this conventional example, data reproduction is realized by recording encrypted data and an encrypting key thereof in the CD-ROM at the time of recording, and decrypting the encrypted data after executing a reading command of the encrypting key from a personal computer to a reproducing device at the time of reproducing. This method has an advantage in that the encrypting key can be easily changed.
Furthermore, as a second conventional example, as shown in FIG. 3 of Japanese Laid-Open Patent Publication No. 7-85574, a system of recording an encrypting key in an area in a disk which the optical head of the reproducing device does not scan was proposed. In this conventional example, since the encrypting key is prevented from being read from a general personal computer, the encrypting key is not copied in a copying operation, and thus an illegal copying operation makes no sense.
However, since the encrypting key of the first conventional example is recorded in the main data field of the sector, it is possible to easily read the encrypting key used at the time of recording the disk from a general personal computer. Therefore, since users can read the encrypting key and the encrypted data, it is highly possible to decrypt the encrypting.
Furthermore, in the second conventional example, the encrypting key is recorded in the area which the optical head of the reproducing device does not scan. In order to read the encrypting key, therefore, reading means dedicated to reading the encrypting key is required, in addition to reading means for reading data from the data recorded area, thus causing a problem.
In both of the first and second conventional examples, data is encrypted with a single encryption key, thereby failing to ensure a sufficient security level in the case where the encryption key is read out by a third party.
The present invention has an objective of providing an information recording medium having a data structure which ensures the prevention of the content recorded in the information recording medium from being illegally copied so as to realize secured copyright protection, an information reproducing device capable of reproducing data from the information recording medium without providing a special data reading means, and solving the first and second problems, and a method for reproducing information.